Conventionally, a metal halide lamp in which mercury or metal halide is enclosed in an arc tube is widely used as a light source for a projection type projector apparatus, which is typified by a DLP etc., that uses a liquid crystal projector or a DMD to uniformly project an image with sufficient color rendering properties onto a rectangular screen.
In recent years, the demand for more miniaturization and realization of a point light source in such a projector apparatus an extra high pressure mercury lamp in which a mercury vapor pressure therein at time of lighting becomes 150 atmospheric pressure or more is mainly used instead of the metal halide lamp. Where such an extra high pressure mercury lamp is used as a light source, since a spread of an electric discharge arc can be suppressed due to very high mercury vapor pressure, it is possible to further improve the optical output.
Such an extra high pressure mercury lamp will be explained below, referring to FIG. 9. The extra high pressure mercury lamp comprises: an arc tube 11 which is made of, for example, quartz glass, and which has a spherical light emission section 12 including a sealed space there inside; and sealing portions 13 which are in a shape of a rod and extend continuously from both ends of the light emission section 12 along with a tube axis thereof, respectively. A pair of electrodes 20 which face each other is arranged in the light emission section 12, so that each electrode 20 is electrically connected, through a metallic foil 30 which is buried air tightly in each sealing portion 13 so as to extend along with the tube axis thereof, with an external lead 15 provided so as to project and extend outward from an outer face of the sealing portion 13. In the light emission section 12 of the extra high pressure mercury lamp, for example, mercury of 0.15 mg/mm3 or more is enclosed, wherein the mercury vapor pressure of the light emission section 12 becomes 150 atmospheric pressure or more at time of lighting.
Since, in the extra high pressure mercury lamp having the above-mentioned structure, the pressure in the light emission section 12 becomes very high at time of lighting, problems occur like the enclosed gas leaks from cracks in the sealing portions 13. In order to solve such problems, it is required that glass which forms the sealing portions 13 be sufficiently and firmly brought into close contact with the respective electrode axis portions 21 and metallic foils 30 for electric supply.
In prior art, for example, in a state where quartz glass, which forms an arc tube making material, is heated at a high temperature, such as 2,000 degree Celsius or more, the sealing portions 13 are formed by gradually shrinking the thick quartz glass, thereby improving the adhesiveness between the quartz glass, and the respective electrode axis portions 21 and metallic foil 30 for electric supply in the sealing portions 13.
However, if the glass was burned at a high temperature, although the adhesiveness between the glass, and the respective electrode axis portions 21 and metallic foil 30 was improved, there was a problem that the sealing portions 13 tend to be damaged after a lamp was built.
This was because, when the temperature of the sealing portions 13 gradually falls after the heating treatment, since the expansion coefficient of, for example, tungsten which formed the electrodes 20 is one or more digit larger than that of, for example, quartz glass which formed the sealing portions 13, cracks occur in contact portions thereof due to relative difference of the amount of expansion of the tungsten and that of the quartz glass. Although the cracks produced at the time of lamp manufacturing were very small in an early stage, they grew up during lamp lighting when the inside of the light emission section 12 became extremely high in pressure. Over time, the cracks became a damage factor for the sealing portions 13 of the lamp.
Although such a problem never occurred when the pressure of the light emission section 12 of the lamp was low, it was a characteristic problem of the lamp in which the inside of the light emission section 12 was high in pressure, such as 150 atmospheric pressure or more, at time of lighting.
The inventors found out that when the high pressure in the light emission section at time of lamp lighting was applied to a gap which was inevitably formed near a joint of an electrode axis portion and a metallic foil, cracks were produced, thereby assisting growth of the gap, so that it was thought that the above-mentioned problems could be solved by making the gap small as much as possible. For example, in Japanese Patent Application Publication No. 2003-257373, an extra high pressure mercury lamp is proposed, in which a metallic foil 100 which has the structure shown in FIG. 8 is used to form a sealing portion.
The structure of the metallic foil 100 of this extra high pressure mercury lamp (FIG. 8) will be explained below. In this metallic foil 100, at the center in a width direction of a strip shaped metal plate (foil making material), a curve groove portion 101, which curves circularly, is formed so as to extend in a longitudinal direction, wherein one end portion of the curve groove portion 101 projects in a longitudinal and outside direction from one end edge of a flat section 105, which is a plate shape and which extends from both side edges of the curve groove portion 101, and extends in a width direction, that is, the full length of the curve groove portion 101 is larger than the full length of the flat section 105.
A base side portion of the electrode axis portion 21 is joined to the projection portion 102 of the curve groove portion 101 of this metallic foil 100 in a state so that an end portion face thereof is located outside the flat section 105 in a longitudinal direction and away from the one end edge of the flat section 105 of the metallic foil 100. In addition, a tip side portion of the external lead 15 is joined to an end portion in the other side of the curve groove portion 101. The Japanese Patent Application Publication No. 2003-257373 teaches that, in the extra high pressure mercury lamp for which the metallic foil 100 of such a structure is used, since the gap which is inevitably produced between the electrode axis portion 21 and the metallic foil 100 (at a position near the joint) can be made small as much as possible, even if the high pressure in the light emission section is applied to the gap at time of lamp lighting, it is possible to prevent generation of the cracks.